Fast PFET Buck Controller
LM51031
Figure 2. Block Diagram
THEORY OF OPERATION
Control Scheme
The LM51031 monitors and the output voltage to
determine when to turn on the PFET. If V
FB
falls
below the internal reference voltage of 1.25V during
theoscillator’s charge cycle, the PFET is turned on
and remains on for the duration.of the charge time.
The PFET gets turned off and remains off during
the oscillator’s discharge time with the maximum
duty cycle to 80%. It requires 7mV typical, and
20mV maximum ripple on the V
FB
pin is required to
operate. This method of control does not its require any
loop stability compensation.
Startup
The LM51031 has an externally programmable soft start
feature that allows the output voltage to come up
slowly preventing voltage overshoot on the output.
At startup, the voltage on all pins is zero. As V
CC
rises,
the V
C
voltage along with the internal resistor R
G
keeps
HTC
−
4
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HTC [ HTC KOREA TAEJIN TECHNOLOGY CO. ]
